1. Field of the Invention
This invention relates generally to production self-noise testing methods for magnetoresistive heads and more particularly to a system for rapidly screening giant magnetoresistive heads for Barkhausen noise during production quality-control testing.
2. Description of the Related Art
In recent times, a class of metallic multilayer films exhibiting the giant magnetoresistive (GMR) effect has gained considerable attention for use as read heads for both hard-disk and magnetic-tape drives. GMR-effect read sensors exhibit a MR ratio, xcex4R/R, which is typically 8% or higher, as compared with 2% for advanced magnetoresistive (AMR) read heads. The increased sensitivity of GMR devices allows a decrease in the read element track width so that track density and, ultimately, storage density can be increased. Today, the exchange-biased spin-valve (SV) is the GMR device attracting the most interest for use as a read sensor. The SV device itself consists of four layers. A free layer and a pinned layer, both comprising a soft ferromagnet, reside on either side of a nonmagnetic copper (Cu) spacer. An exchange layer of antiferromagnetic material (AFM) is deposited next to the pinned layer. The free layer is sufficiently thin to allow conduction electrons to move back and forth frequently between the free and pinned layers via the conducting spacer layer. The magnetic orientation of the pinned layer is fixed and held in place by the AFM layer, while the free layer""s magnetic orientation changes in response to the magnetic field from a bit stored on the surface of a hard disk. This bit-field effectively switches the SV device between two alternative configurations (either high or low resistance), which correspond to the binary datum being read from the disk.
Looking ahead, the tunnel-junction (TJ) device will likely represent the next-generation read head. SVs consist of two ferromagnetic (FM) layers separated by a conductive nonmagnetic spacer layer such as copper, whereas tunnel junctions consist of two FM layers separated by a nonmagnetic electrically insulating layer. Tunnel junctions (like spin valves) exhibit a two-state magnetoresistance dependent upon the relative orientation of the two ferromagnetic layers, which conveniently lends itself to magnetic data-storage applications. Existing tunnel-junction devices exhibit MR ratios of over 25% compared to 10% for SVs. This increased sensitivity allows further decreases in bit size for smaller track widths and larger track densities.
Significant challenges are encountered in the development and manufacture of GMR thin-film devices. Of primary concern is the uniformity and thickness of the deposited layers; the thickness of copper spacing layer is typically less angstroms. Film properties significantly affect the magnetoresistance, resistivity, magnetization, and magnetostriction of the GMR device. Furthermore, intrinsic film properties, such as surface roughness, affect the coupling between layers, the coercivity of the free layer, the effectiveness of the antiferromagnetic layer in pinning one of the magnetic layers, and domain (Barkhausen) noise characteristics.
Barkhausen noise in magnetoresistive heads has been a well-known problem since the late 1970s. Barkhausen noise occurs in any magnetic material when magnetic domains rotate or domain walls move in discrete steps as the ambient magnetic field is varied. The Barkhausen effect consists of discontinuous changes in flux density during smooth magnetic field changes. These are known as Barkhausen jumps (avalanches) and are caused by the sudden irreversible motion of magnetic domain walls as they break away from pinning sites. Barkhausen noise therefore depends on the interaction of domain walls with pinning sites. The dynamics of Barkhausen noise is typical of many complex systems, such as earthquakes, sand-piles, superconductors, and so forth. Magnetic and acoustic Barkhausen noise are both well-known in the art.
Ideally, the MR sensing layers are fabricated as a single domain having no walls to shift noisily during external magnetic field changes. However, the single magnetic domain is sometimes broken by stresses or distortions arising from the head finishing processes. In U.S. Pat. No. 5,926,019, Okumura discloses a method for evaluating, in a frequency range corresponding to actual data recording/playback operation, the Barkhausen noise of a magnetoresistive head after completion of the head-gimbal assembly process. Okumura uses a bandpass filter to remove carrier and harmonics, thereby extracting a single aggregate measure of the Barkhausen noise in a frequency region close to the hard disk data read-back rate. Okumura uses a video display to detect Barkhausen noise and neither considers nor suggests any method for automatic evaluation of the statistical characteristics of Barkhausen noise.
In U.S. Pat. No. 5,721,488, Sakai et al. discloses a method and apparatus for testing an integrated magnetic head assembly for normal operation. Sakai uses the same write head to excite the read sensor for Barkhausen noise measurements but neither considers nor suggests any method for evaluating the statistical characteristics of Barkhausen noise.
In Japanese Patent No. JP10188230, Morita Hiroshi discloses a method for measuring Barkhausen noise in a magnetic head that is fundamentally typical of many earlier methods known in the art. Hiroshi exposes the magnetoresistive head assembly to an external alternating magnetic field such as may be produced by a Helmholtz coil. The magnetoresistive head output is detected, differentiated and filtered by a notch filter to remove the alternating field frequency from the differentiated Barkhausen noise signal. As with earlier methods, Hiroshi neither considers nor suggests any method for automatic evaluation of the statistical characteristics of Barkhausen noise.
Consistent quantification of Barkhausen noise in magnetoresistive heads is a well-known problem in the art. Practitioners such as Hiroshi rely on signal differentiation for quantifying Barkhausen noise. In U.S. Pat. No. 5,854,554, Tomita et al. disclose a method and apparatus for testing magnetic heads that provides for repeated measurement cycles to improve Barkhausen noise detection. Significantly, Tomita et al. teach the use of a visual display monitor as the primary means for detecting the presence or absence of Barkhausen noise. When applied to production quality control, Tomita et al. teach judging whether the tested magnetic head is a good article or not depending upon whether Barkhausen noise is produced or not. This sort of subjective quality control evaluation technique is the only method known in the art for screening production magnetoresistive heads for Barkhausen noise.
There is accordingly a clearly felt need for a magnetoresistive head production quality control screening method that rapidly, reliably and automatically evaluates the statistical Barkhausen noise parameters to judge whether the magnetoresistive head should be accepted or rejected. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.
This invention solves the above problem by rapidly and repeatedly measuring associated noise in a GMR head subjected to a smoothly-varying external transverse magnetic field. The repeated Transverse Magnetic-field Excited Noise (TMEN) measurements are automatically sorted into bins to form a histogram, which is then automatically evaluated to develop TMEN range and weighted sum measures, which are then compared with predetermined standards for automatic acceptance or rejection of the GMR head under test.
It is a purpose of this invention to provide an apparatus and method for the rapid automatic screening of GMR heads for Barkhausen noise at the row level where other quasi-static measurements are performed during production. It is a feature of this invention that the GMR sensor Barkhausen noise is quantified through the use of a bandpass filter to remove all direct sensor responses, leaving only the noise signals, which are then repeatedly sampled to develop valid statistical Barkhausen noise measures suitable for automated analysis.
In one aspect, the invention is a method for measuring one or more intensity parameters of the magnetic noise of a GMR head having a signal bias current including the steps of applying to the GMR head a time-varying transverse magnetic field having a fundamental frequency, filtering the signal bias current to remove all frequency components below a lower band-pass frequency that is substantially higher than the fundamental transverse magnetic field frequency to produce a filtered signal bias current, sampling the filtered signal bias current at a sampling rate substantially higher than the lower band-pass frequency to produce a plurality of digital signals representing a plurality of filtered signal bias current values, sorting the plurality of digital signals by value to form a histogram having a TMEN range and weighted sum, and comparing at least one of the histogram TMEN range and the TMEN weighted sum to a respective predetermined threshold value to determine the intensity parameter of the GMR head magnetic noise.
In a preferred embodiment, the invention is a system for screening a plurality of giant magnetoresistive (GMR) heads for unacceptable magnetic noise levels, including a probe assembly for passing a signal bias current through a first GMR head, a coil assembly for applying to the first GMR head a time-varying transverse magnetic field having a fundamental frequency, a band-pass filter for removing from the signal bias current all frequency components below a lower band-pass frequency that is substantially higher than the fundamental transverse magnetic field frequency to produce a filtered signal bias current, an analog-to-digital converter assembly for sampling the filtered signal bias current at a sampling rate substantially higher than the lower band-pass frequency to produce a plurality of digital signals representing a plurality of filtered signal bias current values, an arithmetic logic assembly for sorting the plurality of digital signals by value to form a histogram having a TMEN range and weighted sum, a comparator for comparing at least one of the histogram TMEN range and weighted sum to a respective predetermined threshold value, and a test data logging assembly for recording the rejection of the first GMR head when at least one of the histogram TMEN range and weighted sum exceeds the respective predetermined threshold value.
The foregoing, together with other objects, features and advantages of this invention, can be better appreciated with reference to the following specification, claims and the accompanying drawing.